Spin and charge drift-diffusion in ultra-scaled MRAM cells

Abstract

Designing advanced single-digit shape-anisotropy MRAM cells requires an accurate evaluation of spin currents and torques in magnetic tunnel junctions (MTJs) with elongated free and reference layers. For this purpose, we extended the analysis approach successfully used in nanoscale metallic spin valves to MTJs by introducing proper boundary conditions for the spin currents at the tunnel barrier interfaces, and by employing a conductivity locally dependent on the angle between the magnetization vectors for the charge current. The experimentally measured voltage and angle dependencies of the torques acting on the free layer are thereby accurately reproduced. The switching behavior of ultra-scaled MRAM cells is in agreement with recent experiments on shape-anisotropy MTJs. Using our extended approach is absolutely essential to accurately capture the interplay of the Slonczewski and Zhang-Li torque contributions acting on a textured magnetization in composite free layers with the inclusion of several MgO barriers.

Promising Materials and Synthesis Methods for Resistive Switching Memory Devices: A Status Review

“…Figure a,c and Figure b,d summarize the formation and dissolution of CF and the nature of I – V characteristics of both unipolar and bipolar devices, respectively. , The next-generation memory devices must be superfast, nonvolatile, and high-density to meet the growing demand of consumer and industrial applications. There are different types of memory devices reposted in the literature such as resistive RAM (RRAM), , magnetoresistive RAM (MRAM), , phase-change RAM (PCRAM), etc. For instance, a ferroelectric tunnel junction (FTJ) based nonvolatile memory device exhibits a unique benefit.…”

Finite Element Approach for the Simulation of Modern MRAM Devices

“…The unconditional convergence of an algorithm coupling the LLG equation with a FE implementation of the spin and charge drift-diffusion formalism was proven in [ 44 ], and the scheme was later successfully applied to metallic spin valves in [ 14 ]. We report here an extension of the scheme to MTJs, which includes the spin dephasing contribution and allows both the TMR effect and the expected torque properties to be reproduced [ 45 , 46 ].…”

“…Dirichlet conditions are applied to prescribe the voltage at the contacts, and the Neumann condition is assumed on external boundaries not containing an electrode, with the unit vector normal to the boundary. In order to be able to reproduce the TMR effect, the tunnel barrier is treated as a poor conductor whose local conductivity depends on the relative magnetization orientation in the RL and FL [ 45 , 46 ]: where and are the Slonczewski polarization parameters [ 11 , 47 ], is the angle independent portion of the conductivity, is the conductivity in the parallel (anti-parallel) state, and is the magnetization of the RL(FL) close to the interface. and are related to the TMR by Julliere’s formula [ 48 ]: where is the resistance in the parallel (anti-parallel) state.…”

“…The inclusion of appropriate boundary conditions at the TB interface with the RL and FL, together with the employment of a low diffusion coefficient inside the TB, allows the expected properties of the torque acting in MTJs to be reproduced [ 46 ]. The additional boundary conditions to be added to the RHS of ( 22 ) read where indicates the interface of the TB with the RL(FL).…”

Accurate Torque Evaluation in Elongated Ultra-Scaled STT-MRAM Devices

“…T S is the spin torque contribution. To get a complete description of the torque contribution which allows to include all physical phenomena in ultra-scaled MRAM operation, a drift-diffusion (DD) model for charge and spin currents is applied (8). Through the steady-state nonequilibrium spin accumulation S, satisfying…”